The importance of roads and highways has been appreciated since the time of the Roman Empire. By about 300 B.C. the first section of the Appian Way extending from Rome to Capua was built. Some of the more than 50,000 miles of roadway ultimately built in the Roman Empire was constructed with heavy stone. However, not much progress was made in the art of road construction from the era of the Roman Empire until the development of the automobile.
For centuries stone blocks, wood blocks, vitrified brick and natural asphalt (bitumen) have been used to pave roads and highways. However, at the beginning of the automobile era, most rural roadway surfacing consisted of broken stone or gravel. Such roads were often rough, dusty and clearly inadequate for modern automobile and truck traffic.
Today the United States has the most extensive highway system in the world with about 2,000,000 miles of paved road. Napoleon realized the importance of roadway systems and built such a system in France which today has the second most extensive system of paved roadways in the world covering about 500,000 miles. Germany, Japan, Great Britain, India, and Australia also currently have systems of paved roads which extend well over 100,000 miles. In addition to these public roadways there are countless paved driveways and parking lots all over the world.
Today roads, highways, driveways and parking lots are often paved with asphalt concrete. Pavement can be made with asphalt concretes which are dust-free, smooth, and which offer the strength required for modern automobile and heavy truck traffic. Asphalt concrete is generally made by mixing aggregate (sand and gravel or crushed stone) with the proper quantity of an asphalt cement at an elevated temperature. The hot asphalt concrete is then placed by a layering machine or paver on the surface being paved and thoroughly rolled before the asphalt concrete mixture cools. The asphalt concrete is normally applied at a thickness varying from about 25 to about 100 millimeters.
Asphalt concrete pavements can be made to be very smooth which offers outstanding frictional resistance for vehicles operating thereon. Such asphalt concrete pavement can also be repaired simply by adding additional hot asphalt concrete to holes and other types of defects which develop in the surface. Asphalt concrete pavements can also be upgraded easily by adding additional layers of hot asphalt concrete to old surfaces which are in need of repair.
Even though asphalt concrete offers numerous benefits as a paving material, its use is not trouble free. One major problem encountered with asphalt concrete pavements is the loss of the adhesive bond between the aggregate surface and the asphalt cement. This breaking of the adhesive bond between the asphalt cement and the aggregate surface is known as "stripping". The stripping of asphalt binder from aggregate surfaces results in shorter pavement life and many millions of dollars of maintenance work on American highways each year. Reduction of this stripping tendency is of great interest when trying to improve conditions of roads and lowering these maintenance costs.
Over the years, various methods have been developed to reduce stripping tendencies. For instance, amines and lime are known to act as anti-stripping agents and are frequently applied to the surface of the aggregate prior to mixing it with the asphalt cement in making asphalt concrete. U.S. Pat. No. 5,219,901 discloses a technique for reducing stripping tendencies which involves coating the aggregate with a thin continuous film of a water-insoluble high molecular weight organic polymer, such as an acrylic polymer or a styrene-acrylic polymer.
U.S. Pat. No. 5,262,240 discloses a technique for providing aggregate with a high level of resistance to stripping by water, which comprises: (1) mixing the aggregate with latex to form a latex/aggregate mixture which is comprised of from about 0.005 weight percent to about 0.5 weight percent dry polymer; (2) heating the latex/aggregate mixture to a temperature which is within the range of about 66.degree. C. to about 232.degree. C.; (3) maintaining the latex/aggregate mixture at said elevated temperature for a time which is sufficient to reduce the moisture content of the latex/aggregate mixture below about 0.7 weight percent and to allow the polymer in the latex to crosslink on the surface of the aggregate to produce the coated aggregate.
At high service temperatures, such as those experienced on hot summer days, asphalt concrete can experience rutting and shoving. On the other hand, at low service temperatures, such as those experienced during cold winter nights, asphalt concrete can also experience low temperature cracking. To combat these problems it is known in the art to modify asphalt cements with rubbery polymers, such as styrene-butadiene rubber latex. Such modification techniques can greatly improve resistance to rutting, shoving, and low temperature cracking. However, the rubbery polymers used in such applications have a tendency to phase separate from hot asphalt cements due to poor compatibility. A solution to the problem of poor compatibility is offered by the technique disclosed in U.S. Pat. No. 5,002,987.
U.S. Pat. No. 5,002,987 relates to a modified asphalt cement containing from about 90 to about 99 parts by dry weight of an asphalt cement and from about 1 to about 10 parts by dry weight of a rubber latex having a weight average molecular weight of less than 250,000 and a Mooney viscosity of less than 50. The latex is a random polymer comprising from about 60 to 100 weight percent of at least one conjugated diolefin containing from 4 to 6 carbon atoms and from about 0 to 40 weight percent styrene. This latex polymer is highly compatible with the asphalt and provides good ductility which results in good resistance to low temperature cracking. However, the utilization of the rubbery polymers described in U.S. Pat. No. 5,002,987 in asphalt cements provide little improvement in elastic recovery or toughness. Thus, their use results in compromised rutting and shoving characteristics. There accordingly is a current need for a modifier which is compatible with asphalt cement and which improves the resistance of asphalt concrete made therewith to rutting, shoving, and low temperature cracking.